The metal strips produced in a rolling mill are normally pickled, particularly after hot rolling and before cold rolling, to remove the oxide coating formed on their surface during the hot rolling process, along with any other deposits or stains that might become encrusted in the sheet metal during cold rolling.
Generally, metal sheets are pickled in tanks containing a hot sulfuric or hydrochloric acid solution through which the sheet metal is passed while immersed in the acid solution. A pickling tank is therefore generally very long, for example, twenty to thirty meters, its width, which remains constant along its whole length, being chosen according to the maximum width of the strips to be treated.
A pickling tank therefore generally comprises a vat made up of a rectangular bottom considerably longer than it is wide, surrounded by a vertical side wall comprising two longitudinal sides and two transversal sides.
The pickling installation generally comprises at least two successive tanks and is located in a large building. Since the vapors released by the acid are liable to corrode any material located above or around the tanks, the pickling tanks are generally located on an upper level of the building, while the other equipment such as the strip collecting means are located on the lower level. For this reason, concrete vats are too heavy to be used, and, in order to provide the required strength without excessive weight, the pickling vat is generally delimited by a wall comprising an external shell made of sheet metal which provides the mechanical strength required, and which is internally lined with a covering that both prevents the acid solution from coming into contact with the metal wall and thermally insulates the wall from the acid solution, which is maintained at a relatively high temperature. The sheet metal is usually lined with a rubber layer which is in turn covered by brickwork comprising at least one layer of aluminosilicate bricks which provides good resistance to acid and satisfactory thermal insulation and which also resists wear caused by occasional or regular rubbing of the steel strip as it passes through the tank.
The vapors relased at the surface of the acid bath are extremely corrosive and can be captured by hoods and directed to the depollution installations. The release of vapors must nevertheless be minimized as much as possible. For this purpose, the tank is closed by a cover which provides as effective a seal as possible on the upper edge of the side wall.
To provide a satisfactory seal between the cover and side wall, a device designated as a "water seal" is generally used. It consists of an upwardly open U-shaped channel filled with water which is fitted along the upper edge of the side wall, and into which penetrates a lip provided on the periphery of the cover.
To date, this channel has also been made of metal and is welded to the upper edge of the sheet metal forming the outer portion of the side wall. While not directly in contact with the acid bath, it must obviously be protected against the corrosive action of the vapors, and is therefore covered with a vulcanized or ebonited rubber covering.
While such an arrangement is theoretically satisfactory, it nevertheless wears fairly rapidly because the rubber covering the channel is subject to considerable shocks which occur when the covers are placed in position, and which, over time, cause the acid-resistant covering to split or even break up. The metal structure is then rapidly attacked and, given the relative ineffectiveness of partial repairs, the upper portion of the tank will need to be completely rebuilt after a few years.
This involves first dismantling the upper part of the brickwork, removing the old channel by cutting the outer shell, and welding on a new rubberized channel.
The welding operation does, of course, damage the rubber lining, which must be removed along a certain length each side of the weld, which in turn requires the removal of bricks. Once the channel has been fitted and welded, the continuity of the rubberizing on the weld must be reestablished and the brickwork rebuilt. This type of repair work takes a very long time, possibly from ten to twenty days depending, for example, on the state the metal sheet is found to be in after the bricks are removed. The installation must be shut down for the entire repair period, leading to a substantial loss of production. Moreover, this kind of seal replacement work must be carried out at regular intervals. To afford better protection for the channel, the use of brickwork has been suggested, but its thickness is limited to approximately 20 mm and it is therefore very fragile. Acid can also seep into the seals.